Sometimes described as "one of the modem cornerstones of our federalism," Erie stands at its narrowest for a simple proposition: When a federal court decides a claim whose source is state law, the court must apply the same substantive common-law rules that a state court would apply to the claim. Dictated by statute, by policy, and by the Constitution, this result seems "superbly right" to many. Indeed, Erie's narrow holding is not controversial today.